Heritability of cognitive performance in aging twins. The National Heart, Lung, and Blood Institute Twin Study

The genetic contribution to performance on scales designed to measure mild to moderate decrements in cognitive functioning in a population at risk is unknown. In the present analysis, 134 monozygotic and 133 dizygotic male twin pairs (mean age, 63 years) were given three cognitive tests: the Mini-Mental State examination, the Iowa Screening Battery for Mental Decline, and, for comparison, the Digit Symbol Substitution Test from the Wechsler Adult Intelligence Scale. The primary objective of the analysis was to test for a significant heritable component to performance on these measures. A secondary objective was to determine the extent to which shared variance with significant confounders such as education, age, and depression affects the outcome of the heritability analysis. Results indicate that performance on tests intended to measure cognitive decline in the elderly does have a significant genetic component and that these estimates tend to increase after adjustment for covariates. Heritability estimates adjusted for covariates were 30% for the Iowa Screening score, 60% for the Mini-Mental State score, and 67% for the Digit Symbol Substitution score.